On my CB400, the right hand side (near side in UK) bottom fork leg, has the brake mounts - see pic.
I know some CB500F's had this as an option to then upgrade to dual front discs, but is this something that was standard on some 400 bikes? My suspicion is that the bike has had a trip down the road at some point and was replaced with a 500 fork leg, but I'm intrigued...
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